1. Field of the Invention
The present invention relates to devices for micro-circuit fabrication. In particular, it relates to lithographic fabrication techniques involving photoresist layers deployed on semiconductor material. More particularly, the present invention relates to the use of Near-Field Optical (NFO) Lithography. More particularly yet, the present invention is a mask system making use of NFO effects to overcome the diffraction limitation otherwise inherent in the use of non-contact masks to direct light patterns onto photoresist material.
2. Description of the Prior Art
The circuit-on-a-chip industry has been characterized since its inception in the 1960s by the production of chips having ever higher device densities. High densities demand high precision in the laying out of the devices and their interconnections on the semiconductor chip. As the densities have increased, so has the degree of precision demanded. For many years, the dominant response to these demands has been to use photoresist-based lithography. This lithography involves "drawing" on the photoresist an image of the circuit or of some portion of it, typically by shining ultraviolet (uv) light on the photoresist through a masking device having slits replicating the desired pattern. (Subsequent steps in the fabrication process then depend on those portions of the photoresist that had been illuminated having different physico-chemical properties than those portions that had not been illuminated.) As a general proposition it can be noted that manufacturing efficiency is improved by investing the effort required to produce a mask and then using that mask to produce large numbers of chips. The more chips that can be manufactured using a given mask, the more industry can afford to invest in a particular mask. Because of this, techniques are available for forming the patterns on the mask that would be impractical if applied directly to the individual chips.
It can be readily inferred that the device density limitations will ultimately depend on how fine the lines are that the manufacturing process can "draw" on the photoresist. For most of the history of the semiconductor industry this ultimate limit was not a concern, since factors other than the fineness of the light pattern established limits that were far grosser. As the drive to higher resolution/density proceeded, however, these factors were resolved one by one; chip fabricators are now faced with the limit imposed by the ultimate resolution of the light patterns with which the photoresist layer can be illuminated.
The limitation that now needs to be addressed is the diffraction-limited resolution attainable from light of a given wavelength .lambda.: .apprxeq..lambda./2. Since at least the time of the Young diffraction experiments in 1802, it has been known that when light is directed onto a slit so as to form an image of the slit on a surface (such as a screen) placed behind the slit, there is a lower limit to the slit width that can be faithfully imaged. As long as the slit is wider than the wavelength of the light shining on it, the image of the slit will be essentially equal to the width of the slit. However, as the slit is reduced to a width on the order of the light's wavelength, the image of the slit will first become noticeably fuzzy along the edges and, as the slit is further narrowed, the image will actually increase in width, while decreasing in intensity per unit area. This effect is often described as arising from diffraction limitations inherent in optical imaging.
In the context of semiconductor chip production, the diffraction limitations of slit imaging can be seen to apply to mask imaging, the means by which a light pattern is directed onto the photoresist layer. The light commonly used in chip fabrication at present is in the near uv, with a wavelength at or somewhat less than 3000 .ANG.ngstroms (.ANG.) .ident.300.times.10.sup.-9 m, 300 nm, 3.times.10.sup.-5 cm, 3.times.10.sup.-4 mm, or 0.3.mu.!. Consistent with this, the narrowest circuit elements occurring in a mass-produced semiconductor chip have a width about 0.1.mu..
Since the "spreading out" of the imaging light occurs between the mask and the photoresist layer, one approach to avoiding the diffraction limitation is to place the mask in direct contact with the photoresist layer. (It is noted in passing that if the illumination system does not result in collimated light falling on the mask, the contact approach would be a necessity.) This "contact mask" approach does exist in the prior art and is discussed in Moreau et al., U.S. Pat. No. 3,676,002, issued in 1972, where it is described as impracticable because of the mask degradation that occurs after a relatively few uses. Moreau et al. teaches the making and use of masks that have uv-transparent material--such as silicon dioxide or a variety of photoresist itself--deposited directly onto their undersides (the sides facing the semiconductor chip) in the areas through which light is to be passed. These deposits are 5000-10000 .ANG. thick and serve the purpose of spacing the masks above the photoresist layer. According to Moreau et al. this approach extends the life of these contact masks, while improving the optical characteristics of the light reaching the photoresist layer on the semiconductor. This patent, however, does not teach the use of this approach in order to achieve resolutions that are finer than the wavelength of the light being used in the process. Furthermore, although extended mask lifetimes are claimed, the masks in question still contact the semiconductor chip, exposing the chip and the mask to damage.
One means of deferring the diffraction limitation is to reduce the wavelength of light directed onto the mask. This quickly becomes impractical, however, because of the increased difficulty in generating the intensity needed as the wavelengths move further into the uv.
An alternative to reducing the wavelength of the incident light is to direct the light through a material possessing a high index of refraction, n, immediately before it encounters the mask's slit pattern. Since materials transparent to uv light and having values of n on the order of 1.6 are available, this approach would in principle lower the diffraction-limited line width by a factor of at least 1.6. In order to implement this approach, however, there would have to be negligible space between the slits and the photoresist layer within which the mask image is to be formed, since once the imaging light emerges from the high-n material in, for example, air, its wavelength would "snap back." The mask/spacer taught by Moreau et al. contains a suggestion of this approach in that within the space between the mask and the photoresist layer the imaging light travels in a high-n material that is directly in contact with the photoresist layer. The spacer is described as a "light pipe" capable of maintaining the light within itself as that light travels from the mask slits to the photoresist. Given the variety of angles with which the diffracted light traveling through the photoresist spacer can strike the walls of the spacer, it appears that not all of that light will be internally reflected. Furthermore, the mask of Moreau et al. is required to contact--through the spacers--the photoresist layer of the semiconductor chip. Finally, Moreau et al. is not concerned with "near-field" effects and their implication for mask imaging.
The statement that the diffraction limitation to slit-imaging resolution had been understood for centuries really holds only for "far-field" imaging, where the imaging surface is more than a few wavelengths away from the source. In the chip fabrication process, the imaging surface is the photoresist layer deployed on the semiconductor chip surface; the source is the underside of the mask. With 3000 .ANG. light, the far-field starts at a distance of a few times 3000 .ANG. from the mask, that is when the distance between the mask and the photoresist layer is on the order of 10000 .ANG. (1.mu.). It is in far-field optics that one can treat the light emerging from a slit as a distribution of plane waves propagating outward from the slit and interfering constructively and destructively with one another in such a way as to form the diffraction-limited image discussed above.
All sources of light, including slits through which light is shone, give rise to homogeneous (propagating) waves and evanescent (vanishing, falling away) waves. Unlike the former, the latter waves decay in amplitude as a function of distance from the source. The decay length depends on the wavelength of the light. These evanescent waves contain "sub-wavelength" information, i.e., information about objects significantly smaller in dimension than the illuminating wavelength. (See, e.g. "An Evanescent Field Optical Microscope," by van Hulst et al. , p. 79, Proceedings American Institute of Physics Conference 241.) Evanescent waves are also associated with total internal reflection of light at an interface between materials with two different values for n. If the light is traveling in the material with the greater value for n, and is incident upon the interface at an angle (measured with respect to the normal to the interface) that exceeds a certain critical value, that light will be totally reflected back into the first material. In spite of the total reflection of the propagating waves under these conditions, there will be evanescent waves present within the second material in the region very near the interface. Should a third material having an index of refraction very close to that of the first material be very close, the evanescent wave can give rise to a propagating wave in the third material. This will happen if the third material lies within the optical near-field of the first material. Similarly, when light waves propagating in a material encounter a good conductor, such as a metal, they will be reflected regardless of the angle of incidence. In particular, they will be reflected even if they are incident perpendicularly to the conductor. Nevertheless, in such a situation, there will again be an evanescent wave present capable of causing a propagating wave to occur in material adjacent to the conducting interface if the conducting material is sufficiently thin, as, for example, in the case of a thin metal film, and the other material is very close to the film.
If the source-image separation is considerably less than the wavelength being used for imaging, one says that the imaging is occurring in the "near-field" region, or, with the wavelength falling within the optical range, the near-field optical region. In contrast to the long-established understanding of far-field imaging, near-field imaging has until recent years received little attention. See, for example, NEAR-FIELD OPTICS: Theory, Instrumentation, and Applications, by Paesler, M. A., and Moyer, P. J., Wiley-lnterscience, 1992. Near-field optical techniques have within the past few years been applied to microscopy and other types of visualization, where the goal is to be able to "see" objects that are much smaller than the wavelength of the light used for illuminating the object. In such instances, it is the light reflected from a surface (e.g., microscope slide) that is of interest, in contrast to lithography, where it is the light that is incident on a surface (photoresist layer) that is of interest. There are, among the prior art, several recent patents directed at the use of near-field optical effects in microscopy. See, for example, Kino et al., U.S. Pat. No. 5,004,307, which describes near-field and solid immersion optical microscopy. The microscopy invention taught by Kino et al. is very similar to the lithography system described in Corle et al., which relates to lithography system embodying a solid immersion lens.
Many of the benefits of placing the mask directly in contact with the photoresist can be achieved, while avoiding the detriments, by placing the mask extremely close but not in direct contact. Extremely close means to within a distance much less than the wavelength of the light illuminating the mask, so that the photoresist layer lies within the "near-field" region of the light emanating from the underside of the mask. Furthermore, this technique can be combined with making the mask out of a material with a high index of refraction so that the wavelength of the light reaching the mask pattern is reduced from that of the illuminating light. This involves manufacturing the mask containing entity from a material with a high index of refraction, n. One can think of this entity as a vertically-oriented cylinder, the top end of which is illuminated by the external light which then passes through the cylinder and out the bottom face, where the bottom face contains the pattern to be imprinted on the photoresist.
As stated, the first use of near-field optical techniques was in observational microscopy, for high-resolution viewing of an object, as opposed to a high-resolution imprinting of an object. The problems to overcome when one is using light to view a surface at high resolution are very different from those that arise when one wishes to use light to create high-resolution patterns on a surface. For one thing, in the latter situation but not in the former it is necessary to tightly control scattered light, so that it does not get on those parts of the surface that are not supposed to be illuminated. In particular, successful lithography using near-field optical techniques requires a means of limiting unwanted wave propagation.
Accordingly, the prior art fails to provide any proximity mask device that utilizes uv light to image a light pattern in photoresist with dimensions much smaller than 0.3.mu. for fabrication of integrated-circuitry. In particular, the prior art fails to provide a simple proximity mask that addresses the problem of evanescent light leakage onto the photoresist layer. Therefore, what is needed is a simple proximity mask device that provides a control means to prevent evanescent wave propagation to photoresist areas that are not to be illuminated.